OBJECTIVE: To determine the prevalence of human T-cell lymphotropic virus type I (HTLV-I) infection among patients with lymphoproliferative disorders, as well as among their family members and sexual contacts, at two sentinel sites in Cuba. METHODS: An analysis was conducted of all the patients with a presumptive diagnosis of hematological malignancies seen by the hematology departments of the Hospital Hermanos Ameijeiras (HHA), City of Havana, and the Hospital Provincial Comandante Faustino Pérez (HPCFP), Matanza, Cuba, in January 1996-January 1997. HTLV-I seropositivity was determined by ELISA and Western Blot, and infection was confirmed by polymerase chain reaction. The positive patients' family members and sexual contacts were also assessed. The Z-test was used to compare proportions. RESULTS: Seroprevalence of HTLV-I infection in patients with lymphoproliferative disorders was 0.4% higher at the HPCFP than at the HHA (6.1% versus 0.2%, P < 0.001). There were no significant differences in prevalence by age, sex, or skin color. Of the 53 family members and sexual contacts studied, 8 (15.1%) were positive for HTLV-I infection. CONCLUSION: The prevalence of HTLV-I in the study group was higher than previously found in Cuba. The value of seroepidemiological surveillance through sentinel sites was confirmed.